Theory of Spin Transport in Silicon Nanostructures

University Of Rochester, Rochester NY

Investigators

Abstract

Future technologies beyond the CMOS age are becoming exceedingly important research frontiers. In this regard, scientists conjectured that spin-based devices must be on the horizon. In these devices, the quantum mechanical spin is introduced to electronic circuits. Silicon is an ideal material choice for spintronic devices. It has a mature technology which overwhelmingly dominates the semiconductor industry. Importantly, silicon has a relatively long spin relaxation time due to its weak spin-orbit coupling and the suppression of important spin relaxation mechanisms (via the crystalline inversion symmetry and the zero nuclear spin of its naturally abundant isotope). Accordingly, spin information can be carried over 1 mm length scales in silicon. Intellectual Merit of the proposed activity: I. Studying the spin relaxation in silicon devices in the presence of strain, electrical fields and quantization. In this part, the basic device geometry includes a transport channel between spin injector and detector. Transport across this basic structure will be studied using a drift-diffusion model whose basic parameters (e.g., spin lifetime and charge mobility) will be calculated from rigorous numerical methods that accurately quantify the electron-phonon interaction. These include an empirical pseudopotential method and a k∙p model to calculate the electron wavefunctions and an adiabatic bond-charge model to calculate the phonon dispersion and atom displacement vectors. Strain effects can be readily incorporated in these models while quantization effects along the confinement direction are explicitly considered in the k∙p model. The electron-phonon scattering will be modeled by a rigid-ion approximation. Group theory will be used to identify selection rules that simplify the analysis. II. Incorporating the gained knowledge in device simulations. Spin injection, extraction and the role of contact widths will be carried by realistic boundary conditions taken from verified experimental results. A new concept for on-chip spin-based information transfer that alleviates much of the acute problems of on-chip transmission lines will be studied. Heating effects of drifting conduction electrons (by the electric field) in silicon wires and silicon-germanium nanostructure devices will be investigated by Monte Carlo simulations which quantify both the charge transport and spin lifetime. Other than spin flips, momentum relaxation will be modeled by electron-lattice and electron-impurity scattering. Broader impacts of the proposed activity: The proposed research includes new theoretical aspects and device concepts in silicon spintronics. This research is timely and may have an important impact since spin injection in silicon has recently become a very active research field. Other than publications in peer-reviewed journals and presentations in professional conferences, outcomes of this project (theoretical predictions and device model results) will be shared with leading experimentalists in silicon spintronics. The broad impact of our previous spintronics results has been highlighted in several science magazines including the Science News, Nanomaterials News and the New Scientist. Educational activities are an essential part of this project and are closely integrated with the proposed program. The proposed research presents ideas that are fascinating to a wide range of students. By collaboration of engineering and physics graduate students, each side learns from the other quantum mechanics or device function concepts. The PI has already designed a new course on spintronics which explains how quantum mechanics and magnetism can be applied into new logic and memory architectures. The PI is committed to training students and will integrate this research with undergraduate courses and will recruit qualified undergraduate students to participate in the research. The PI will actively promote this research among K-12 students by delivering interactive lectures/demos that highlight the use of future technology in devices. The potential of this research for high societal impact will hopefully attract students from under-represented groups to the project.